The present invention relates to a switching power device for an audio amplifier which comprises a low frequency power transformer, eliminates the respective drawbacks of a power supply and a switching power supply and has both advantages.
As a power device for an audio amplifier, conventionally, there have been used a power device utilizing a low frequency power transformer and a switching power device (for example, see Patent Documents 1 and 2).    Patent Document 1: JP-A-7-274388 Publication    Patent Document 2: JP-A-5-176532 Publication
When designing a power supply for an audio amplifier, it is necessary to take care of the following respects.    1. The audio amplifier is to reproduce a voice at a lower limit (20 Hz) of a voice frequency or less. For this reason, it is necessary to provide a capacitor having a large capacity in the output of the power supply for the audio amplifier.    2. It is desirable that the audio amplifier should satisfy that at least a certain degree of an output is a power harmonic regulating object.    3. The audio amplifier is to pass a temperature test and the temperature test sets, as a rated output, an output in which a distortion rate reaches a specific value when all channels are driven for approximately one minute at the same time and is decided depending on whether an output obtained by multiplying the rated output by a specific coefficient is equal to or smaller than a reference value.    4. The audio amplifier usually has a high SVRR (an in-phase voltage removing ratio). For this reason, there is no problem if a ripple included in the output of a power supply for the audio amplifier is within a limit.
Accordingly, a power device designed to satisfy each of the contents is used for the conventional audio amplifier.
However, there are the following problems in the design of the conventional audio amplifier. More specifically, in the case in which a power device comprising a low frequency power transformer is used, there is a problem in that many restrictions are imposed in respect of a design when a low frequency power transformer is used as a small-sized integral type power supply for a multichannel audio amplifier because the volume of each low frequency power transformer is large. When a low frequency power transformer having a small size is used in order to eliminate the restrictions in respect of the design, moreover, a load regulation is deteriorated. For this reason, it is necessary to cause the breakdown voltage of a capacitor to be higher. Furthermore, the low frequency power transformer is an a stable power supply and is influenced by a line regulation and a load regulation. For this reason, it is necessary to cause the breakdown voltage of a capacitor having a large capacity on a secondary side to be higher. In addition, when the copper loss of the low frequency power transformer is decreased to enhance the load regulation, there is a problem in that a power factor and a power harmonic are deteriorated. When a power device comprising a low frequency power transformer is used as a power supply for a multichannel audio amplifier which can reproduce a surround voice, moreover, it is impossible to obtain a value by adding an output in the driving operation of only one channel corresponding to all channels in a full-channel simultaneous output due to a load regulation characteristic, and there is a property that an output voltage is reduced in proportion to an output current. There is a problem in that this property is hard to control optionally.
On the other hand, in the case in which a switching power device is to be used, a high frequency power transformer has a small copper loss. For this reason, a power harmonic is easily generated and a rush current is increased. Therefore, it is necessary to take countermeasures against the power harmonic and measures for preventing the rush current. Moreover, the switching power device has an excellent load regulation at a constant voltage output and an output power is increased in proportion to a load current. For this reason, when a switching power device is used as a small-sized integral type power supply for a multichannel audio amplifier, a value is obtained by adding the output in the driving operation of only one channel corresponding to all of the channels in the full-channel simultaneous output. Consequently, the output of an amplifier becomes excessive. If there is such a property, moreover, a temperature test for the audio amplifier is carried out at a large output. Therefore, tough conditions are set.
Although an output is practically carried out instantly for each channel in the audio amplifier, however, all of the channels are rarely driven at the same time. If the total output of the audio amplifier in the full-channel simultaneous output has the value obtained by adding outputs in the driving operation of only one channel corresponding to all of the channels as described above, however, there is no advantage to a user and the temperature test becomes uselessly strict. It is preferable to positively restrict the output in the full-channel simultaneous driving operation in the same manner as in the case in which a power device comprising a low frequency power transformer is used.